Effects Of Stress Amplitude Research Articles

Effect of stress amplitude and stress duration on twinning and phase transformations in shock-loaded and cold-rolled 304 stainless steel

The residual microstructure of 304 stainless steel has been examined by transmission electron microscopy following shock loading to pressures of 150, 250 and 350 kbar at a constant pulse duration of 2 μsec, and 0.5, 1.0 and 6.0 μsec at a constant pressure of 250 kbar. These observations were compared with those for the same material cold-rolled to reductions in thickness of 15, 30 and 35%. Deformation twins were observed in all deformed samples. The twin volume densities increased for increasing shock pressure at constant pulse duration, and for constant pressure for pulse durations of up to 2 μsec α′-martensite was observed as trace microstructure at 2 μsec (250 kbar) while at 6 μsec the volume percent of α′-martensite was observed to be roughly 5%. α′-Martensite in the cold-rolled samples was observed to increase steadily in volume percent with increasing reduction by rolling. Residual microhardness was also observed to increase steadily with increased reduction by rolling, while residual microhardness increased with pulse-duration at 250 kbar of up to 1μsec, then decreased to a hardness plateau between 2 and 6 μsec. The residual deformation twin structure in all samples is shown to consist of fault bundles composed of a varying composition of stacking faults, ϵ-phase and twins, with a small contribution of ϵ-phase in shock-loaded microstructures, except at 6 μsec (250 kbar). The results confirm the formation of α′-martensite within these fault bundles and at intersecting twin faults in both the shock-loaded and cold-rolled 304 stainless steel.

The Effect of Stress Amplitude below the Fatigue Limit on Cumulative Fatigue Lives in Perforated Round Specimens

The fatigue lives under the service load are usually determined by the stress amplitude over the fatigue limit, but several experiments performed hitherto have showed that the stress amplitudes below the fatigue limit, which take the greater part of the service load, influence the fatigue life. But there is not any systematic experiment reported on this problem. Therefore authors have performed program fatigue tests by rotating bending with perforated round specimens of normalized 0.35% carbon steel under the load spectra, which include a stress amplitude below fatigue limit. To clearify the effect of these low stress amplitudes, the length of the fatigue crack is measured. As for the program fatigue lives in this experiment, modified Miner's law can be used for the estimation. According to the results of the measurement of fatigue cracks, the effect of the stress amplitude below fatigue limit changes depending on the crack length at the time the stress is applied, the magnitude of the stress amplitude, and the number of stress cycles in a program cycle.

The Effect of Stress Amplitude on Fatigue Processes of Annealed and Cold-Rolled Low-Carbon Steels

The specimens of 0.01%C annealed and 15% cold-rolled steels were fatigued under completely reversed plane bending. The effect of stress amplitude on fatigue process of these materials was investigated by using the X-ray microbeam diffraction technique, optical microscopy and electron microscopy. The results are summarized as follows:(1) The stress cycles at slip band formation Ns and the crack initiation Nc were measured of both the materials by means of an optical microscope. The ratios of Ns and of Nc to the number of stress cycles at fracture Nf were obtained of the annealed material asNs:Nc:Nf=0.02∼0.04:0.40:1, and of the cold-rolled material they are expressed asNs:Nc:Nf=0.002∼0.003:0.26:1.The critical stress amplitude of slip band formation during fatigue in annealed materials is nearly equal to the frictional stress.(2) The amount of crystal deformation during fatigue was dependent on the applied stress amplitude. In the case of the annealed material, the micro-lattice-strain Δd/d at the half of the total fatigue life was related to the stress amplitude σn in the following equation, α·σn=9.3+2.0×104(Δd/d), where α is the stress concentration factor of the specimens 1.03.The excess dislocation density D is also a function of stress amplitude. The function is approximated as follows:α·σn=11.0+5.8×10-4√D, when σn is less than 22kg/mm2.(3) For the cold-rolled material, total misoriention β at the half of the total fatigue life increased when stress amplitude was higher than the yield stress. Subgrain size decreased during fatigue and the amount of its decrease was larger when σn was higher.(4) Fatigue cracks were nucleated through the linking of pores formed along the subboundaries in the annealed material. A similar mechanism of fatigue crack initiation by pore linking was verified for low stress amplitude fatigue in the case of the cold-rolled material. On the other hand, the formation of pores was not observed in high stress amplitude fatigue.

Dynamic Strain Amplitude During Fatigue Process of Mild Steel due to Rotary Bending

Annealed mild steel specimens have been subjected to rotation under the bending stress of 21∼31kg/mm2 with the speed of stress repetition at 3∼3000rpm at room temperature to examine its dynamic strain amplitudes and creep strains. A wire strain gage and an ocillogram have been employed by means of a slip ring to record the process.(1) Fatigue process, which is expressed by the dynamic strain amplitude, can be divided into three stages: first stage of softening, second stage of hardening, and third stage of apparent softening.(2) When the speed of stress repetition is slow and the stress amplitude is high, the amplitude of dynamic strain increases to a larger value, and the effect of stress amplitude on the specimens is larger than that of the speed of stress repetition.From the experimental results, it has been made clear that fatigue life is in inverse proportion to the peak of dynamic strain amplitude, i. e. the maximum value of softening process.(3) With the exception of the third stage, the dynamic strain amplitude on fatigue process is proportional to the transient creep strain during fatigue process.

Closure to “Discussions of ‘Effect of Stress Amplitude on Statistical Variability in Fatigue Life of 75S-T6 Aluminum Alloy’” (1953, Trans. ASME, 75, pp. 870–871)

Closure to “Discussions of ‘Effect of Stress Amplitude on Statistical Variability in Fatigue Life of 75S-T6 Aluminum Alloy’” (1953, Trans. ASME, 75, pp. 870–871)